The present invention relates to thick film electronic circuits and systems and methods for creating such circuits using lead free solder.
Thick film electronic modules are an extremely important family of electronic products, especially in the automotive industry. Thick film technology has become indispensable in the automotive environment since thick film electronic circuits can withstand higher temperatures characteristic of the automotive operating environment and since the values of the conductors and resistors may be very precisely controlled. Many automotive applications utilize thick film products such as mass air flow sensors which operate in the extremely harsh environment of a vehicle""s engine compartment.
A thick film electronic circuit is typically created by first applying inks to a ceramic substrate using silk screening or similar printing methods. Conductive traces, resistors, and dielectrics may be formed from the conductor, resistor, and dielectric inks, respectively, on the ceramic substrate. An ink for the conductor is typically made of conductive particles, typically made of metals or alloys, mixed with other additives in a suspension. These additives serve to facilitate the deposition of the ink onto the substrate, typically by printing, and the adhesion of the conductor to the substrate after drying and firing. Similarly, resistor and dielectric inks are typically made of resistive particles and dielectric particles, respectively, mixed with other additives for the same purpose of facilitating deposition of the ink and adhesion to the substrate. Typically, materials such as alumina (Al2O3) are suitable ceramic substrate materials. After, for example, the conductor ink is printed on the substrate according to the circuit design, the substrate is then dried and fired in an oven to form the conductor trace on the substrate. Resistors and dielectric elements are formed similarly using resistive and dielectric inks, respectively, on the substrate. Solder paste, typically made of particles of solder alloys in a flux vehicle, is then deposited (by screen printing) onto conductor pads (which are integral with the conductive circuitry on the substrate), and then electronic components are placed on the conductive pads. The component/substrate assembly is then placed in a reflow oven having a prescribed temperature-time profile. During the reflow process, the solder melts and interacts with the conductor and component terminations. Thereafter, the solder cools down and solidifies to form an interconnect between the conductor and the component, forming the electronic module. Generally, the solder is an alloy primarily containing tin and lead. Typically, tin-lead solder having compositions of 63% tin; 37% lead or 62% tin; 36% lead; 2% silver (all percentages are by weight) is used.
Unfortunately, while prior art systems and method for creating thick film circuitry achieve their intended purpose, significant problems still exist. For example, the lead (Pb) in the tin (Sn) lead solder is known to be extremely toxic to the environment. To overcome this concern, much effort has been expended to develop a lead-free solder alloy. Presently, the automotive industry has selected a tin-silver (Ag) alloy having the composition of 96.5% tin and 3.5% silver by weight; various variations of the Snxe2x80x94Ag alloy, such as tin-silver-copper (Cu) (for example 95.5% Sn, 3.9% Ag, and 0.6% Cu by weight), may also be used.
Currently, there are primarily two types of conductors used to create thick film electronic circuits. One type is made of palladium silver (Pdxe2x80x94Ag) and the other type is made of silver (Ag). Typically, palladium-silver conductors having approximately, a one to three palladium to silver. ratio and having other minor additives are used. The palladium-silver conductor has. been a typical choice for thick film products. Unfortunately, extensive metallurgical interdiffusion occurs between the Pdxe2x80x94Ag conductor and the tin based solder. This occurs primarily due to the existence of Pd in the conductor. Such metallurgical interdiffusion has a serious adverse impact on the reliability of the interconnection. For example, if extensive interdiffusion occurs between the solder and the conductor, adhesion between the solder, conductor, and ceramic substrate is severely compromised. Under normal thermomechanical loading, which occurs during normal vehicle operation, cracks are initiated and propagate through the interconnect leading to electrical failure.
One prior art solution has been to print and reflow a high lead solder (such as 10% tin; 90% lead or 10% tin; 88% lead; 2% silver by weight) as an interdiffusion barrier layer between the conductor and eutectic tin lead solder. Unfortunately, it is not viable to interconnect a component directly with the high lead solder since the high lead solder""s melting temperature requires a high soldering temperature that could severely damage the components rendering the electronic circuitry inoperable. Further, while the high-lead barrier layer solves the interdiffusion problem, this method may not be used in lead free solder applications because of the high (90%) lead content.
Other prior art solutions to address the problems stated above have been to use silver as a conductive material. Silver has been shown to not induce extensive interdiffusion with the tin based solder due to the absence of palladium in the conductor. An added benefit is that a silver conductor is less costly than the palladium silver conductor (palladium is very expensive and is in short supply, major sources of palladium exist in Russia). One significant problem, however, exists using a silver conductor. Silver reacts with sulfur, which is present in the automotive environment. Sulfur converts the silver into silver sulfide which is nonconductive. The end result is electrical failure and the circuit is seen as an open, again rendering the electronic module inoperable. Significant numbers of warranty returns have been experienced in the automotive industry due to silver sulfide formation.
Therefore, there is a need for a new and improved system and method for attaching electronic components to a thick film circuit. Such a new and improved system and method must address toxicity concerns of using a lead-based solder, must reduce or eliminate metallurgical interdiffusion, and reduce or eliminate silver sulfide formation when using a silver conductor.
The present invention overcomes the problems stated above and other problems not addressed by the prior art by providing a system and method for attaching electrical components to thick film circuits.
In an embodiment of the present invention, a silver conductor pad is provided where solder paste will be deposited and electronic components placed. Further, to prevent silver sulfide formation, the silver conductor pad will be completely covered with solder by over-printing the pad with solder. Thus, no silver will be exposed to react with sulfur, thereby preventing silver sulfide formation. A palladium-silver conductor material is used for the rest of the conductor traces, thus also preventing silver sulfide formation. Since the palladium-silver conductor is not used for the solder pad area, no solder/conductor interdiffusion will occur.
Preferably, in another embodiment of the present invention, the conductor traces will have a thickness of 5-30 microns.
Preferably, in another embodiment of the present invention, the solder paste is 96.5% tin and 3.5% silver (by weight) thus eliminating the use of lead. Further, the solder paste is printed with a thickness of 50-300 microns. Once the solder paste is printed, the electronic components are placed onto the solder paste. The populated assembly is then processed in a reflow oven having a prescribed temperature-time profile.
Preferably, in yet another embodiment of the present invention, the reflow temperature-time profile will include a peak temperature of 225-280xc2x0 C., will have a dwell time of about 20-150 seconds at 221xc2x0 C., and a ramp rate (heating/cooling) of below 10xc2x0 C. per second.
In still another embodiment of the present invention, the atmosphere of the reflow process may be ambient air. However, in yet another embodiment of the present invention, a nitrogen atmosphere is preferable and will lead to more beneficial results.
Thus, the present invention eliminates the need for lead based solder for thick film electronic products, prevents failures due to extensive interdiffusion, and eliminates silver sulfide formation. Moreover, the present invention eliminates the need for a high-lead barrier layer and thus the associated process complexity, cost, and toxicity concerns.
Further objects, features and advantages of the invention will become apparent from consideration of the following description and the appended claims when taken in connection with the accompanying drawings.